The present invention relates to an assembly for reducing the exhaust gas emissions of an internal combustion engine, in particular for an automotive vehicle combustion engine, including a catalytic converter.
As has been proposed in the prior art, catalytic converters of combustion engines are provided with electrical heating means for heating the catalytic converter at cold start, and with a thermally insulated exhaust gas tube between the engine and the catalytic converter, in order to reduce the high exhaust gas emissions at cold start which result from the catalytic converter having not yet reached its operative temperature necessary for a proper functioning thereof.
In order to minimize the expenditure of the electrical heating of the catalytic converter and to reduce heat losses through the exhaust gas conduit, the catalytic converter should be positioned as close as possible to the engine. This, however, will be in conflict with excessive heat situations where the extremely hot exhaust gases may detrimentally affect the catalytic converter.
In order to meet these conflicting requirements German patent application 23 03 773 proposes to divide the exhaust gas conduit between the engine and the catalytic converter into a pair of conduit branches of substantially different lengths, with the longer branch extending in a loop so as to allow for sufficient cooling of the exhaust gases at full load, even though the catalytic converter is positioned relatively close to the engine. This design suffers from substantial space requirements for the loop-shaped conduit branch.
This is why German patent application 42 18 834 proposes to position the catalytic converter remote from the engine such that excessive heating at cold start will be avoided and to divide the exhaust gas conduit between the engine and the catalytic converter into a pair of conduit branches being of the same length, however, of different cross-sectional areas, with the branch of smaller cross-sectional area receiving all of the exhaust gas when the catalytic converter is cold. The purpose of this design is to feed the exhaust gas to the still cold catalytic converter while it has not been cooled significantly. This is so because the exhaust gas flows through the small cross-section of the used conduit branch at a higher speed than it would flow through the total cross-sectional area of both branches of the exhaust gas conduit. However, in this prior design both conduit branches are in heat exchange relationship with the ambient air so that thermal energy of the exhaust gas be directly transferred to the ambient air via the conduit walls.